1. Field of the Invention
The present invention generally relates to the art of microelectronic circuits, and more specifically to a gallium arsenide Monolithic-Microwave-Integrated-Circuit (MMIC) flip chip or other microelectronic circuit structure including inorganic dielectric passivation layers for supporting interconnecting conductor bridges and encapsulating the surface of the structure.
2. Description of the Related Art
The flip chip configuration is highly desirable for the fabrication of MMIC microelectronic circuit structures since it eliminates the need for wirebonds or other leads to interconnect a MMIC chip on a ceramic (for example Low-Temperature-Cofired-Ceramic (LTCC)) substrate. A MMIC flip chip includes metallic (e.g., silver or gold) plated electrical contact bumps which extend from input-output (I/O) and electrical bias sites on the chip surface for mating with the contacts on the ceramic substrate.
The chip is inverted and placed in contact with the ceramic substrate such that the bumps are aligned and pressed against solder dots formed on the contacts. Heat is applied to cause the solder to reflow and fuse the bumps to the contacts to provide a permanent ohmic contact.
A description of MMIC flip chip technology is presented in a paper entitled "Flip Chip Manufacturing Technology for GaAs MMIC", by W. S. Wong et al, 1993 U.S. Conference on GaAs MANufacturing TECHnology, May 1993, Atlanta Ga.
GaAs MMICs are currently passivated by a thin (2,000 angstrom) silicon nitride film, which provides limited protection for the active layer regions of GaAs FETs, and forms MIM capacitors for MMIC circuits. However, interconnects such as transmission lines and airbridges are exposed to ambient air, especially so for high-power MMICs.
Moisture can easily become trapped in corners of metal lines and beneath airbridges. In the case of flip chips, these exposed metal features are vulnerable to damage during the solder reflow mounting process and other assembly and handling steps. Chemicals such as solder flux, organic solvents and photoresist residue can cause short circuits and long term microwave module reliability problems.
Gold leaching and shorting of interconnect metallizations and airbridges can occur during the solder reflow process if solder contacts the chip surface. Gold can also be leached from contact bump posts due to wicking of solder from the bumps to bump posts. In addition, circuit elements on MMIC chips can be damaged during the solder defluxing process by the solvents used to remove the flux.
Solder dots can be applied to contact sites on a metallized ceramic substrate by silk screening. However, the lateral dimensional tolerance of this process is insufficient for MMICs with exposed metallizations due to ragged edges which can be formed on solder dots when the silk screen is removed, and which can cause short circuits to the metallizations on MMIC chips.
For this reason, dabbing is conventionally used for applying solder to ceramic substrates on which MMICs are to be mounted. In this expensive and time consuming method, each solder dot is formed individually by dabbing a precise amount of solder onto the substrate from an applicator. Although precise lateral dimensional tolerance can be achieved using this method, the thickness or height of the solder areas cannot be controlled accurately. The height of the bumps on the MMIC flip chips must be sufficiently precise to make up for this deficiency, adding to the complexity and expense of the fabrication process.
More importantly, existing GaAs flip chip MMICs do not provide sufficient hermetic environmental protection for moisture and mobile ions, which are major contributors to corrosive, electro-chemical and other circuit failures.